Transcript of "J227984"

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International Journal of Engineering Science Invention
ISSN (Online): 2319 – 6734, ISSN (Print): 2319 – 6726
www.ijesi.org Volume 2 Issue 2 ǁ February. 2013 ǁ PP.79-84
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Crustal Deformation along the Manipur Hills Segment of
Indo Myanmar Ranges of the Northeast India
Sanasam Subhamenon Singh
Pravabati College, Mayang Imphal, Manipur, India
ABSTRACT: Crustal deformation along the Indo Myanmar Ranges (IMR) is due to the subduction of Indian
plate below the Myanmar micro plate. Shortening across the ranges calculated along the profiles perpendicular
to the regional trend varies gradually with a maximum of 59% approximately. Shortening accommodate by the
minor local scale folding is less than the regional shortening. The amount of excess shortening is
accommodated by a series of imbricate thrust system in the lower horizon of the Disang sediments, and folding
of the upper horizon of the Barails, develop due to the tectonic compression. A shear couple stress field
produced by the relative plate motion is responsible for the crustal deformation around the Manipur Hills
segment of the IMR.
Keywords: Barails, Disangs, imbricate, shortening, thrust.
I. INTRODUCTION
The Indo-Myanmar ranges (IMR), evolves as an accretionary prism along the subduction margin of the
Indian plate below Myanmar micro plate of Eurasian continent. The general trend of the hill ranges follows a
curvature, NE-SW in the northern part to NW-SE in the southern part. Axes of the major folds are also
following the same trend. Many regional scale thrust are found parallel to the regional trend as well as parallel
to each other. The degree of compression is found to be more on the northern side, gradually reduce southward.
This variation in the degree of compression is examined by calculating the amount of shortening, along different
lines across the range perpendicular to the regional trend. Balance cross-section techniques are applied for
calculating the shortening across the ranges perpendicular to the general trend.
Also it seems to be evident that crustal deformations and tectonics of this region is directly related to
the motion of the Indian plate with respect to the Myanmar micro plate. As reveal by the data from various
sources, Indian plate is moving towards north east direction. Myanmar plate is moving northward with
transcurrent movement along Sagaing fault. Due to the configuration of the margin of the overriding Myanmar
plate, and the motion direction of the Indian plate, a resultant compression along the direction, approximately in
the east-west, is produced, as well as a dextral strike slip couple may be set up. This dextral couple can be used
to explain the structural lineaments associated with the rocks and other topographic features in this region.
Further, by calculating the shortening percentage across local scale minor structural features like folds,
comparison can be made with that of the regional scale shortening.
Subduction of the Indian plate below the Myanmar plate has been observed by Fitch (1970 [1],
1972[2]). Motion of the Myanmar micro plate has been described by Curray et al (1978) [3]. Earliest regional
geological accounts of Manipur Hills in brief were provided by Pascoe (1912) [4] and Evans (1964) [5].
Systematic geological maping of the region was carried out by Dayal (1963) [6], Dayal and Daura (1963) [7].
Accounts of regional geology and tectonics of northeast India are found in the works of Chattopadhyay et al
(1983) [8], Ghosal (1983) [9], Mitra et al (1986) [10], Saxena (1987) [11] etc. Crustal deformation and tectonic
shortening in the region have been estimated by using the principles of balance cross section given by
Dahlstrom (1969) [12], adopted by Coward (1992) [13] and Soibam (1993) [14].
An attempt is made in the present work to provide a brief account of the structural and tectonic framework of
the region, with an objective of understanding the evolution mechanism of the IMR basin.
II. REGIONAL GEOLOGY
Indo-Myanmar ranges extend in between 93o
E to 95o
E longitudes and in between 19o
N to 27o
30’ N
latitudes. The margin of the eastern foothills is supposed to be aligned with the Myanmar plate boundary
overriding above the Indian plate. The rocks are made up of Neogene molasses, tertiary and cretaceous Flysch
sediments with minor igneous and metamorphic rocks of older age. The general stratigraphic sequence of the
IMR is given in Table I. The whole ranges is longitudinally divided into Four major tectono-stratigraphic units,
namely, Metamorphic Belt, Ophiolite Belt (Ophiolite Mélange Zone), Central Flysch Belt, and Molasses Belt.

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The metamorphic complex on the extreme eastern part is composed of low to medium grade phyllitic schist,
quartzite, marble, granite-gneiss etc and is the oldest in the stratigraphic sequence, but over thrust the younger
sequences westward. The eastern ranges contains the Ophiolite mélange zone, approximately 200 km long
Ophiolite suit, chiefly made up of basic and ultra basic intrusive and extrusive, associated with conglomerates,
volcanic pillows, coarse grained exotic sandstones, pelagic sediments like shales, chert, and limestones. This
ophiolite belt in the western side of the metamorphic belt is thrusting over the central flysch belt of Disangs and
Barail sediments. Surma sediments lie in the westernmost part of this region in contact with the Barail, and
further extend with younger Tipam sediments. General lithological and structural lineaments are trending NNE-
SSW. The Metamorphic unit is exposed only in a small part in the Ukhrul District of Manipur, but exposed well
in Saramati and Nimi formations in Nagaland, and Somra Tract of Myanmar (Brunschwieller, 1966 [15];
Acharya et al, 1986 [16]; Bhattacharya & Sanyal 1985 [17];). Ophiolite rocks, mainly peridotite ultramafic,
with serpentinites and associated pelagic sediments like limestones, chert, shales, and exotic blocks of coarse
grained sandstones, together constituted the Mélange zone. Within the Flysch Belt, The older Disang formation
overlies the younger Barail sediments along the Churachandpur-Mao thrust; however, synclinal remains of the
same Barail beds are seen frequently on top of the ranges. Surma sequences of the Molasses sediments are
found in the western part of the ranges, overthrusted by Barails. Beyond, the younger sequences of Surmas and
Tipam extend towards west as usual. The median line of the IMR is a major water divide in between the
catchments of Chindwin-Irrawady system in the eastern side and catchments of Brahmaputra-Ganga system in
the western side. This median range runs along the Arunachal-Myanmar boundary, in its northern end, trending
NE, crosses the Indo-Myanmar international boundary through Tuensang district of Nagaland, passing
Zunheboto and Phek districts entered Manipur on the eastern side of Senapati district and runs along the Kenedy
peak of Chin Hills segment in Myanmar.
III. CRUSTAL DEFORMATIONS ACROSS THE MANIPUR HILLS SEGMENT
Structural and tectonic features in the Manipur Hills segment of the IMR infer an east-west tectonic
compression with shortening of the region. The degree of shortening is expressed as the percentage ratio of the
amount of shortening upon the original length. Method used by Coward (1992) [13], subsequently adopted by
Soibam (1996) [18], has been applied. During the deformation of the rock beds, due to lateral pressure, we
assume three conditions that, rocks are incompressible, change in the width (the other horizontal perpendicular)
is negligible and the amount of shortening is compensated by vertical thickening only. Two commonly used
techniques are i) Line balancing (Gibbs, 1990 [19]; Hossack, 1979 [20]; Woodward et al, 1989 [21]), and ii)
Area balancing (Chamberlain, 1910 [22], 1919 [23]; Bucher, 1933 [24]; Goguel 1962 [25]; Dahlstrom, 1969
[12]; Laubscher, 1976 [26]; Hossack, 1979 [27]; Coward, 1992 [13]; Soibam, 1996 [18]),.
In the first case, a visible stratigraphic section profile is used to stretch out one of the bedding to its original
position and original length is calculated. The second case uses only a topographic profile. Area beneath the
curve and a selected datum line is calculated, which is further used for calculation of the original width of the
whole region before compression.
Topographic profiles are drawn across selected sections, which are almost parallel to the direction of
the resultant compression. Endpoints or pin lines are chosen in such a manner that both end of the profiles fall in
between the same tectonic segment. The foreland on the western front of the range system has an altitude below
50 m of the MSL. Therefore, mean sea level is taken as datum line for calculating the area. The thickness of the
crust below the IMR varies from 52 to 55 km. And the thickness of the crust outside the mountain belt is that of
the oceanic crust under the Bay of Bengal, which is taken approximately, 20 km (Rajsekhar & Mishra, 2008
[28]; Mukhopadhyay & Dasgupta, 1988 [29]; Mukhopadhyay and Krishna, 1991 [30]). Hence the approximated
excess thickness is taken as 33 km. By using these values, shortening across the sections are calculated along
some chosen profile lines. The eastern ends of the profiles are taken along the eastern foothills of the range
system and the western margin is chosen along the line of Barak-Makru Rivers. (A tectonic break is inferred
along this line). Table II shows the amount of shortening across different section along the IMR. A mount of
shortening is found as maximum along the sections through the Mount Iso Tolloi peak and Siroih peak.
3.1. Regional compression and shear couple
Motion of the Indian plate with respect to Eurasian plate is described by its pole of rotation at 24.4 N,
17.7 E, with an angular velocity () 5.310−7
deg yr −1
moving along NNE direction (around N18), subducting
below the Eurasian plate (De Mets, et al, 1990 [31]; Soibam, 2006 [32]). For Myanmar plate, it is 23.86o
N,
125.12o
E, with  6.410−7
, moving towards N 350o
(Curray, et al, 1978 [3]; De Mets, et al, 1990 [31]; Soibam,
2006 [32]). The direction of relative plate motion is therefore, along the NE-SW, and the compression direction
is EW. Relationship between the motion direction and compression direction is explained by the shear coupled
stress, with the help of strain ellipse deformation mechanism (Soibam, 1996 [18], 2001 [33]). The compression
features like thrust and folds are aligned along the direction 45o
from the shear couple orientation; extension

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features like normal faulting, extension joints are perpendicular to compression direction. Synthetic and
antithetic strike slip features are along directions making 60o
with the extension directions. The shear couple is
produced by the combination of relative motion direction, configuration of the overriding margin of the
Myanmar plate and variable rate of motion of the plate pairs. The tectonic features observed in the Manipur
Hills segment are aligned according to this mechanism.
3.2. Deformation structures
In Manipur Hills segment of the IMR, regional thrust and folds are well aligned with the compression
direction of the region. Amount of shortening in this segment of the IMR seems to be accommodated well along
the thrust planes. Shortening accommodated by folding is comparatively less. Shortening calculated from minor
folds observed in the field (Table III) and inferred folds from the section profile constructed from field data
(Table IV) are compared with the regional shortening to examine the amount of shortening shared by the
structural features.
These minor folds are produced by the east-west tectonic compression and mostly on the upper layers
(Barails and Upper Disangs). The amount of shortening accommodated by them is less than the degree of
regional shortening of the profiles where they exposed (5, 6, and 7 of Table II). The remaining amount of
regional shortening may be accommodated in the thrust zones, parallel to the regional trend. When the regional
shortening ranges from 40 to 60 percent (averaged to 50 %), shortening contributed by the folding ranges from
31 to 35 percent (averaged to 33%). These folds are in the Central Flysch belt, in between two major thrust
zones, the Ophiolite thrust, and Churachandpur Mao thrust. Inferred folds from the section profiles constructed
from bedding data are all from the Barail beds. They are synclinal remains on top of the hill ranges and occurred
in between major thrust planes. Shortening accommodated by them is also very less compared to the regional
one.
3.3. Depth to detachment
The degree of shortening shared by the local folds as well as inferred fold bands in between major
thrust planes are always less than the degree of regional shortening along the profile section nearby it. Depth to
detachment of the deformed segments is calculated from the section profiles assuming a plain strain condition
(Woodward et al., 1989 [21]). Around Manipur Hills segment, the depth to detachment is increasing towards
the zone of maximum shortening, indicating concentration of stress (Table V).
3.4. Joints and fractures
Plot of 564 joints data, mostly from the flysch belt, shows that the general elongation direction is
almost NS. Conjugate pairs of joints are also well represented in the rosette. Rose diagram drawn with the
Georient Software is given in the Fig. 1.
IV. DISCUSSION
It is observed that folding accommodate comparatively more amount of shortening in the sections
southward. Probably, Barail beds are folded on top of the imbricate thrust slices propagated through the
underlying Disang beds. Lower horizon of the Disangs, as observed in the riverbed exposures in the Manipur
Hills segment, has approximately NS trending strike dipping easterly at high angles, 70o
to 85o
. Again width of
around 40 km of the Central Flysch belt has continuous exposure of Disangs sequences. It is not possible for
Disangs, having this much thickness. Average thickness of Flysch sediments are given only 5 to 6 km (Muang,
1987 [34]; Acharya, 1991 [35]; Nandy, 1980 [36]). The wide exposures of the Disangs might be probably due to
repeated overlapping of the imbricate slices. Sediments in the lower horizon are subject to vertical pressure
exerted by the overloading sediments and horizontal compression is accommodated by thrust systems, following
ramp-flat geometry model given by Twiss and Moores, (1992) [37]; Woodward, et al., (1989) [21]; Suppe
(1983) [38]; Boyer & Elliote (1982) [49]. Upper layers are folded with sharp anticlines along the thrust planes,
which were prone to erosion and occupied by streams and major rivers. New thrust planes are propagated
westward (Fig. 2).
V. CONCLUSION
Crustal deformation along the IMR is due to tectonic compression produced by the relative motion of
Indian and Myanmar plates. Relative motion direction, configuration of the Myanmar plate margin, and variable
rate of motion generate a shear couple, to produce the compression stress along approximately EW direction
with NS extension. Most of the compression stress is accommodated by imbricate thrust system in the lower
horizon of Disangs and by folding in the upper horizon of Disangs and Barails.